High-purity, fully water-soluble alkali metal nitrates or phosphates are particularly useful in horticulture and have wide application in various industries such as in the manufacture of pharmaceuticals, food or feed. While various methods have been proposed in past for their production only few have been commercialised.
Potassium nitrate, the third most widely used potassium salt in agriculture is traditionally produced from an ore containing sodium nitrate, potassium nitrate, some chlorides, and sulphates. The application of this technology is, however, limited by the availability of the nitrate ore.
Potassium nitrate can also be produced synthetically in a low temperature reaction of potassium chloride with nitric acid followed by extraction of the coproduct hydrochloric acid with an organic solvent. Bringing a volatile organic substance in contact with nitrate may be hazardous, and recovering the solvent will have an impact on the performance and economy of the process. Hydrochloric acid, especially together with nitric acid, is highly corrosive and introduces serious limitations to the equipment construction materials. Furthermore, in the absence of a local need hydrochloric acid must be considered as a waste.
Ion exchange technology has also been proposed for potassium nitrate production. In this process the hydronium ions from nitric acid are exchanged with potassium ions from potassium chloride, giving a potassium nitrate solution and a hydrochloric acid solution, see U.S. Pat. No. 5,110,578. A drawback with this “direct” ion exchange process is the risk for mixing potassium chloride and nitric acid whereby a very corrosive fluid will be formed (aqua regia).
At present, most of the potassium phosphate salts used in industry and agriculture are produced from pure raw materials, potassium hydroxide or carbonate and purified phosphoric acid. Potassium phosphates are excellent fertilisers and much research is done in an effort to find an economical production process based on cheap raw materials and obtain an acceptable product quality.
The production of monopotassium phosphate from lower grade raw materials, potassium chloride and wet process phosphoric acid, has been investigated intensively during the last years, U.S. Pat. Nos. 4,836,995; 4,885,148; and 5,114,460. In all processes described in these three patents the real challenge is the separation of chlorine from potassium. In these processes this is done either by evaporation or by solvent extraction of the by-product hydrochloric acid. Direct evaporation of hydrochloric acid is problematic due to the formation of insoluble potassium phosphate compounds, which will reduce the overall yield. In the organic solvent extraction process the recovery of the solvent is essential for the overall economy and also for avoiding organic material in the waste stream.
In U.S. Pat. No. 4,678,649 a process is described for the manufacture of pure monopotassium phosphate without utilising solvents to remove the hydrochloric acid. According to the process, monopotassium sulphate is reacted with a phosphate constituent selected from phosphate rock, dicalcium phosphate or mixtures thereof in the presence of phosphoric acid. The outputs of the process are gypsum, calcium phosphate, hydrochloric acid, and monopotassium phosphate. Mixing sulphuric acid with potassium chloride produces monopotassium sulphate at an elevated temperature whereby hydrochloric acid will evaporate, which will limit the selection of construction materials. Hydrochloric acid and the significant amounts of gypsum generated in the process may be regarded as waste.
Ion exchange technology has also been considered in the production of fertilisers and especially in connection with the production of chlorine-free potassium salts, see U.S. Pat. Nos. 3,993,466, 4,008,307, and 4,704,263.
In the potassium phosphate production process described in U.S. Pat. No. 4,008,307 the raw materials are phosphoric acid and potassium sulphate. The ion exchange process can be either cationic or anionic. In both cases the output will be a potassium phosphate solution and a sulphuric acid solution. An organic solvent is needed to extract potassium phosphate from a sulphate containing solution.
In U.S. Pat. No. 4,704,263, which is dealing with a cationic process for producing potassium phosphate, the ion exchange feed streams are a metal phosphate salt solution and a potassium chloride solution. The metal phosphate salt may be calcium phosphate, magnesium phosphate or iron phosphate, and more particularly, a monocalcium phosphate. A drawback of using monocalcium phosphate is the need of phosphoric acid and the low concentration of calcium ions in the solution, necessitating a calcium enrichment step in the continuous ion exchange carousel system (ISEP).